you can cite the sources that you have in a document like essays, research paper, in several formats. MLA is one of the formats you can cite your sources in.
MLA stands for Modern Language Association of America
In high school and mostly in college and universities a lot you are required to cite the sources that you have used. If you don't cite your sources you will be called to have plagiarised which is sort of like presenting someone else's idea as your own kind of like cheating.
The most preferred format of citing sources is MLA.
You can get more information about MLA from the official website of MLA which is http://www.mla.org/
Easybib is the most popular and common online software used to cite sources easily and it is very time saving.
http://easybib.com/
Sources:
I learnt about all this in the beginning of high school in 9th grade English and other classes.